This invention relates to a process for saturating an oxygenate containing stream to destroy substantially all oxygenates. Moreover, this invention relates to saturating essentially all olefins in the oxygenate containing stream.
Processes for the oligomerization of light olefins to produce heavy olefin oligomers are known. Oligomerization processes have been long employed to produce high quality motor fuel from C4 olefins. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products.
In the oligomerization method of the indirect alkylation process set forth in, for example, U.S. Pat. Nos. 6,080,903 B1, 5,990,367 B1 and 5,895,830 B1, light aliphatic olefins such as C4 olefins are contacted with solid phosphoric acid (SPA) catalyst in the presence of a heavy paraffin diluent such as cyclohexane or isooctane. The presence of the paraffin diluent is believed to promote the oligomerization in the liquid phase to yield predominantly dimerized oligomers such as C8 olefins. Light paraffinic feed can be dehydrogenated to provide the feed for the indirect alkylation process. Patents disclosing such dehydrogenation include U.S. Pat. Nos. 4,393,259 B1, 5,049,360 B1, 4,749,820 B, 4,304,948 B1 and 2,526,966 B1.
Other oligomerization processes using an ionic exchange resin catalyst to oligomerize light olefins to produce oligomers such as C8 olefins are also known. These processes often include an oxygenate such as tert-butyl alcohol (TBA) or sec-butyl alcohol (SBA) in the feed for modifying the catalyst to maintain desired selectivity. When water is added to feed containing C4 olefins, it reacts with C4 olefins to generate TBA and SBA which moderate the resin catalyst. References disclosing resin catalyzed oligomerization include U.S. Pat. No. 5,877,372 B1 and EP 0994088 A1. A dehydrogenation zone to convert paraffinic feed into olefinic feed can also precede the resin catalyzed oligomerization.
The oligomerization of C4 olefins across an acidic catalyst such as SPA produces a variety of C8 olefins. When the feed contains only isobutylene, the predominantly produced olefin is 2,4,4-trimethylpentene which can be saturated to 2,2,4-trimethylpentane. Normal butenes present in the feed oligomerize to produce C8 olefins such as 2,2,3-trimethylpentene and dimethylhexenes. Secondary reactions can also occur in which 2,4,4-trimethylpentene product isomerizes through a methyl shift to give 2,3,4-trimethylpentene or 2,2,3-trimethylpentene and dimethylhexenes. Additionally, C4 olefins can also trimerize to produce triisobutane after saturation.
Table 1 gives the octane numbers and boiling points for C8 isomers and for C12.
Product oligomers with higher octane numbers and lower boiling points are the most preferred. Accordingly, 2,2,4-trimethylpentane is the most preferred C8 isomer; whereas, 2,3,4-trimethylpentane and 2,3,3-trimethylpentane are the least preferred C8 isomers because of their high boiling point temperature. The dimethylhexanes are undesirable because of their low octane numbers and relatively high boiling points and triisobutane is undesirable because of its very high boiling point temperature.
U.S. Pat. Nos. 5,856,604 B1, 5,847,252 B1 and 5,811,608 B1 teach succeeding such oligomerization processes with a saturation reaction zone to convert heavy oligomeric olefins into heavy alkanes that can be blended with gasoline stock. Saturation is known to be particularly beneficial when saturating isooctenes to isooctane gasoline. These patents disclose using a catalyst comprising a metal from a top row of Group VIII of the Periodic Table of the Elements such as nickel and a metal from Group VI-B of the Periodic Table of the Elements such as molybdenum in the saturation reaction zone. However, they indicate no appreciation of the effectiveness of such catalysts with respect to oxygenates and the criticality of properly preparing the catalyst.
One must be careful to avoid catalysts for saturating oligomerized olefins to make gasoline fractions that can lower octane ratings and excessively lower the boiling point down to the range of C4 hydrocarbons. For instance, saturation catalysts can crack C8 hydrocarbons down to C4 hydrocarbons which degrades product quality by lowering the octane rating. On the other hand, cracking of C12 or heavier hydrocarbons down to lighter hydrocarbons in a saturation unit would be desirable. Saturation catalysts that isomerize C8 hydrocarbons to a C8 isomer that has a lower octane number and/or higher boiling point are less desirable.
Saturation processes are typically required to meet certain high level of conversion of olefin to alkane. For example, future gasoline specifications are expected to require a very low level of olefins in the pool. The saturation process must be able to sufficiently convert isooctenes to meet these requirements. Moreover, if unreacted pure isobutene is subjected to saturation for recycling in a process where isobutane is oxidized to obtain tert-butyl hydroperoxide, the recycled isobutane must contain less than 100 ppm isobutylene. In addition, in such a process, the isobutane stream may contain no sulfur and essentially no oxygenates. Hence, high conversion in the saturation process is very important.
U.S. Pat. No. 4,244,806 B1 discloses hydrogenating a stream of isobutene dimers and trimers over a nickle or noble metal catalyst on a non-acidic carrier such as alumina. However, noble metal catalysts are sensitive to the presence of oxygenates and sulfur compounds in the hydrogenation stream. Oxygenates are compounds that contain oxygen such as alcohols, ethers, ketones, aldehydes or water. Notable sulfur compounds include mercaptans and dimethyl disulfides. To avoid diminishing noble metal catalyst activity, a hydrogenation feed may have-to be purified of sulfur through a sulfur unit, thereby increasing the capital cost required to operate with a noble metal catalyst. Furthermore, the hydrogenation feed may have to be purified of oxygenates by running it through at least one oxygenate removal unit (ORU) containing adsorbent to adsorb the oxygenate. Such ORU""s must be shut down periodically to regenerate the adsorbent requiring installation of an additional ORU or substantial down time. Additionally, a large volume of regenerant for regenerating the adsorbent must be obtained and disposed of periodically. The elimination or substantial reduction of oxygenates is becoming more important because of recent governmental regulations to decrease or eliminate- the use of methyl tert-butyl ether (MTBE) as a gasoline blending component and heightened environmental concern over the effects of oxygenates. Methods of removing oxygenates will become more useful because oligomerizing C4 olefins over resin catalyst can produce as much as 5 wt-% C8 ethers.
Hence, it is an object of the present invention to provide a saturation process that uses a catalyst that can withstand sulfur and oxygenates in the feed. It is a further object of this invention to provide a saturation process that uses a catalyst that essentially saturates all olefins. It is a still further object of the present invention to provide a process that destroys all oxygenates and converts sulfur compounds into easily managed hydrogen sulfide. It is an even further object of the present invention to provide a saturation process that minimally degrades product quality.
We have discovered a process that utilizes a catalyst comprising a top row Group VIII metal and a Group VI-B metal, such as a nickel-molybdenum catalyst, to completely destroy all oxygenates, essentially completely saturates all olefins and converts all sulfur compounds to hydrogen sulfide which can be removed in an overhead of a distillation column, thereby excluding sulfur compounds from the gasoline pool. Additionally, we have found that a sufficiently sulfided nickel-molybdenum catalyst has sufficient activity but with sufficiently low acidity to optimize cracking and minimize isomerization to obtain high quality saturate product.
In one embodiment, the present invention relates to a process for producing heavy alkanes from light olefins. The process comprises passing a feed stream containing light olefins to an oligomerization zone. The feed stream is contacted with an oligomerization catalyst to produce heavy olefins. An oligomerization effluent from the oligomerization reactor containing heavy olefins and oxygenates is passed to a saturation reactor. The oligomerization effluent is contacted with hydrogen in the presence of a saturation catalyst comprising a metal from a top row of Group VIII of the Periodic Table of the Elements and a metal from Group VI-B of the Period Table of the Elements in the saturation reaction zone to produce heavy alkanes. A saturation effluent is recovered from the saturation reaction zone containing less than 100 ppm of oxygenates.
In another embodiment, the present invention relates to a process for saturating olefins comprising feeding a saturation stream containing olefins and oxygenates to a saturation reaction zone. Hydrogen is fed to the saturation reaction zone. The saturation stream is contacted with a saturation catalyst comprising a metal from a top row of Group VIII of the Periodic Table of the Elements and a metal from Group VI-B of the Periodic Table of the Elements. An effluent is recovered from the saturation reaction zone comprising less than 100 ppm of oxygenates.
In a further embodiment, the present invention relates to a process for deoxygenating a stream containing oxygenates. The process comprises feeding hydrogen and a stream containing oxygenates to a deoxygenation zone. The feed stream is contacted with a catalyst comprising a metal from a top row of Group VIII of the Periodic Table of the Elements and a metal from Group VI-B of the Periodic Table of the Elements. An effluent is recovered from the deoxygenation zone comprising less than 100 ppm of oxygenates.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.